Patent GB-810423-A (GB)
Title: Apparatus for testing a characteristic of ferromagnetic materials
IPC: G01R33/12
Filed: 1955-06-10
Abstract: 810,423. Electric and magnetic tests; moving-coil measuring instruments. VILLAMOSIPARI KOZPONTI KUTATO LABORATORIUM. June 10, 1955 [June 11, 1954], No. 16835/55. Class 37. Apparatus for non-destructively testing a characteristic of a ferro-magnetic material comprises a first measuring unit deriving a first electric signal dependent on the crosssectional area of the material, a second measuring unit setting up a homogeneous alternating magnetic field, means for constraining flux lines to occupy a predetermined configuration in the second measuring unit and associated with path means guiding the material through the two measuring units, so that the second unit derives a second electric signal dependent on the desired characteristic, a member for compensating variations in the second signal due to crosssectional diversities in the material in accordance with the first signal, and means for indicating the second signal as corrected by the first. In Fig. 4, the material 45 is fed through guiding means 41 and 81 surrounded by measuring units 40 and 80. The former may be as in Specification 790,429, and comprises two coils magnetically linked to a predetermined length of the material by a yoke of low reluctance. One coil is energized from a current source 43 so that the material within unit 40 is saturated. The voltage induced in the other coil and available on leads 92 is then proportional to the crosssectional area of the material. Unit 80 also comprises two coils magnetically linked to a predetermined length of the material by a yoke of low reluctance. One coil is energized by circuit 93 to produce a magnetic field in the material. The voltage induced in the other coil and available on leads 94 is then proportional to the total flux in the material, i.e. the induction times the cross-sectional area. For measuring the permeability in Fig. 4, the cross-sectional area signal on leads 92 is fed to a corrector member 84, controlling circuit 93, so that the field strength set up in unit 80 is inversely proportional to the crosssectional area. The signal on leads 94 proportional to the total flux in the material is thus proportional to the permeability, but independent of the cross-sectional area, and is indicated cn an indicator 82. For measuring the specific core loss, circuit 93 energizes the current coil of a wattmeter 820, and circuit 94 energizes the voltage coil thereof. It is assumed that the core loss is proportional to the mass of material within unit 80 (i.e. to the cross-sectional area) and proportional to the maximum induction in the material raised to the power of 1.7, and therefore (since the induction is proportional to the field strength which is inversely proportional to the cross-sectional area by the correcting action of member 84<SP>1</SP>) inversely proportional to the cross-sectional area to the power of 1.7. The core loss which would be indicated by the wattmeter 820 is thus corrected to the specific core loss by a factor equal to the cross-sectional area to the power of 0.7 by feeding the crosssectional area signal on leads 92 to a third coil in a corrector device 84<SP>2</SP> (described later) in wattmeter 820. In the above embodiment, the field strength set up in unit 80 is corrected according to the cross-sectional area, so that the signal on leads 94 is corrected. As an alternative, the field strength may be made constant, and the indications depending on the signal on lead 94 may be corrected for errors due to crosssectional diversities by suitable members such as 84<SP>2</SP> incorporated in indicators 82<SP>1</SP> and 820. Fig. 15 shows a modification in which a distributer 430 energizes duplicate units 40 and 40a over circuits 90, 90a and energizes an oscillator 95 and amplifier 96 over circuit 90<SP>1</SP>. Amplifier 96 energizes the unit 80 over circuit 93, and the latter and the output circuit 94 energize the current and voltage coils of a wattmeter 820 for measuring the specific core loss as in Fig. 4. To correct, however, for variation in the crosssectional area, the signal in leads 92 representing the cross-sectional area of the material 45 as measured by unit 40 is combined with the signal on leads 92a representing the crosssectional area of a reference piece 45a as measured by unit 40a, in a unit 97. The crosssectional error signal on leads 92<SP>1</SP> is passed through a resistance-capacity time delay filter 98 to a corrector member 84 incorporated in the wattmeter 820. The time delay is chosen equal to the time taken for a given portion of the material 45 to pass from unit 40 to unit 80. By these means a more accurate correction for cross-sectional diversities is obtained. A feedback voltage from circuit 94 is fed over lead 940 to the amplifier 96 to ensure a sinusoidal waveform in circuit 93. The wattmeter 820 may have a second current coil energized in the opposite sense to that connected to circuit 93, to balance out the power losses occurring in the magnetic yoke of unit 80 and in the wattmeter itself, which would otherwise be indicated with the losses in the material 45. Similar modifications may be made to the permeability measuring circuits in Fig. 4, for example by using the signal on leads 92<SP>2</SP> (Fig. 15) to control the corrector 84<SP>1</SP> (Fig. 4) instead of the signal on leads 92. The wattmeter 820 and corrector member 84 are shown in axial section in Fig. 16 and in section on line XVII-XVII in Fig. 17. Two fixed current coils 104 are wired in series with circuit 93, so that their fields reinforce each other. The voltage coil 103 is mounted on a rotatable two-part shaft 137<SP>1</SP>, 137<SP>2</SP>, urged to a datum position in which the axes of coils 103, 104 are at right angles by weak coil springs 138<SP>1</SP>, 138<SP>11</SP> which also serve as current leads for the coil 103. The power consumption is indicated by the deflection of shaft 137<SP>1</SP>, 137<SP>2</SP>, which is preferably small and indicated by a mirror 141 co-operating with a beam of light and a scale in known manner. The shaft portions 137<SP>1</SP> and 137<SP>2</SP> are electrically conducting, being secured together by an insulator 139 and serving as leads for a coil 136 secured to the shaft. When the shaft is in its datum position, the axis of coil 136 is aligned with the magnetic field set up by a permanent magnet 142, 143<SP>1</SP> and 143<SP>11</SP> and two soft-iron members 144<SP>1</SP>, 144<SP>11</SP> supported by a non-magnetic member 145. A damping frame 146 is also secured to shaft 137<SP>1</SP>, 137<SP>2</SP>. Thus while the meter indicates zero power consumption, current flowing in coil 136 has no effect, but if the shaft is deflected, coil 136 will exert a torque proportional to the current flowing in it, and, for small angles, to the deflection.